Method for manufacturing a welded component with very high mechanical characteristics from a coated lamination sheet

ABSTRACT

Plate consisting of a steel substrate ( 1 ) and a precoat ( 2 ) consisting of a layer of intermetallic alloy ( 3 ) in contact with said substrate, topped by a layer of metal alloy ( 4 ), characterized in that, on at least one precoated face of said plate, an area ( 6 ) situated at the periphery of said plate has said metal alloy layer removed.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuing application of U.S. applicationSer. No. 12/293,898, filed on Nov. 25, 2008, which is the national stageof international application PCT/FR2007/00536, filed on Mar. 29, 2007,and claims the benefit of the filing date of PCT/FR2006/000898, filed onApr. 9, 2006, each of which is incorporated by reference herein in itsentirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT

Not Applicable.

REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAMLISTING APPENDIX SUBMITTED ON A COMPACT DISC AND ANINCORPORATION-BY-REFERENCE OF THE MATERIAL ON THE COMPACT DISC

Not Applicable.

BACKGROUND OF THE INVENTION

Field of the Invention

The invention concerns the fabrication of plates or blanks of coatedsteel intended to be welded and then heat treated to obtain parts havinggood mechanical characteristics and good corrosion resistance.

Some applications require steel parts combining high mechanicalstrength, high impact resistance and good corrosion resistance. Thistype of combination is particularly desirable in the automotive industrywhich requires a significant reduction in vehicle weight and excellentcapacity to absorb energy in the event of a collision. This can beachieved in particular by using steel with very good mechanicalcharacteristics having a martensitic or bainitic-martensiticmicrostructure: anti-intrusion, structural or safety components ofautomotive vehicles such as bumpers, door reinforcements, B-pillarreinforcements or roof reinforcements, for example, require the abovequalities.

Description of the Related Art

Patent EP 0971044 discloses a fabrication method in which hot- orcold-rolled steel plate coated with aluminum of aluminum alloy is thestarting material. After shaping to produce a part, and before heattreatment at a temperature above A_(c1), the coating is heated to form asurface alloy by interdiffusion between the steel and the aluminumcoating. This alloy prevents decarburization of the metal and oxidationduring heat treatment in a furnace. It therefore eliminates thenecessity for furnaces containing a special atmosphere. The presence ofthis alloy also obviates certain surface operations on the treatedparts, such as shot blasting, which operations are necessary for plateshaving no coating. The parts are then cooled under conditions adapted toconfer a tensile strength that can exceed 1500 MPa.

With the aim of reducing vehicle weights, parts have been developedconsisting of steel blanks of different compositions or differentthicknesses continuously butt-welded together. These welded parts areknown as “butt-welded blanks”. Laser beam welding is a preferred methodof assembling such blanks, exploiting the flexibility, quality andproductivity characteristics of the process. After these welded blankshave been cold-pressed, parts are obtained having mechanical strength,pressability, impact absorption properties that vary within the partsthemselves. It is therefore possible to provide the required propertiesat the appropriate location without imposing an unnecessary or costlypenalty on all of the parts.

The fabrication method described in patent EP 0971044 can be applied tobutt-welded blanks in the following manner: starting from steel plate,possibly of different compositions or thicknesses, and having a metalpre-coating, butt-welded blanks are obtained by a welding process. Thesewelded blanks then undergo heat treatment to form a surface alloy andare then hot-pressed and quenched. This produces quenched parts withthicknesses and intrinsic mechanical characteristics that vary andrepresent an ideal response to local loading requirements.

However, this fabrication method runs into considerable difficulties:when welding coated steel blanks, a portion of the initial surfacepre-coating is transferred into the molten area created by the weldingoperation. These exogenous metal elements are concentrated in particularby strong convection currents in the liquid metal. These elements aresegregated in particular in the interdendritic spaces in which theliquid fraction having the greatest concentration of dissolved elementsis located. If austenizing follows with a view to quenching the weldedblanks, these enriched areas become alloyed through interdiffusion withthe iron or other elements of the matrix and form intermetallic areas.On subsequent mechanical loading, these intermetallic areas tend to bethe site of onset of rupture under static or dynamic conditions. Theoverall deformability of the welded joints after heat treatment istherefore significantly reduced by the presence of these intermetallicareas resulting from welding and subsequent alloying and austenizing.

It is therefore desirable to eliminate the source of these intermetallicareas, namely the initial surface metal coating liable to be meltedduring butt-welding. However, eliminating this source itself gives riseto a serious problem: the precoated area on either side of the futurewelded joint can be eliminated, for example a mechanical process. Thewidth of this area from which the pre-coating is removed must be atleast equal to that of the future area melted by welding so as not toencourage subsequent formation of intermetallic areas. In practice, itmust be much more than this, to allow for fluctuations in the width ofthe molten area during the assembly operation. Thus there exist afterthe welding operation areas on either side of the welded joint that nolonger have any surface metal pre-coating. During further alloying andaustenizing heat treatment, scale formation and decarburizing occurwithin these areas located next to the weld. These are areas that tendto corrode when the parts go into service because they are not protectedby any coating.

There is therefore a requirement for a fabrication process that preventsthe formation of intermetallic areas within welded assemblies, which aresources of the onset of rupture.

There is also a requirement for a fabrication process such that thewelded and heat treated parts have good corrosion resistance.

There is also a requirement for an economic fabrication process that canbe integrated without difficulty into existing welding lines and that iscompatible with subsequent pressing or heat treatment phases.

There is also a requirement for a product on which operations ofbutt-welding, then of heat treatment, pressing and quenching, lead tothe fabrication of a part having satisfactory ductility and goodcorrosion resistance. One particular requirement is for a totalelongation across the welded joint greater than or equal to 4%.

An object of the present invention is to solve the problems referred toabove.

BRIEF SUMMARY OF THE INVENTION

The invention therefore consists in plate consisting of a steelsubstrate and a precoat consisting of a layer of intermetallic alloy incontact with the substrate, topped by a layer of metal alloy. On atleast one precoated face of the plate, an area situated at the peripheryof the plate has the metal alloy layer removed.

The precoat is preferably an alloy of aluminum or based on aluminum.

The metal alloy layer of the precoat preferably comprises, by weight,from 8 to 11% of silicon, from 2 to 4% of iron, the remainder of thecompound being aluminum and inevitable impurities.

The width of the area from which the metal alloy layer has been removedis preferably between 0.2 and 2.2 mm.

The width of the area from which the metal layer has been removedpreferably varies.

The thickness of the intermetallic alloy layer is preferably between 3and 10 micrometers.

The area from which the metal alloy has been removed is preferablyproduced by partly eliminating the metal alloy layer on at least oneprecoated face of the plate by brushing.

The area from which the metal alloy has been removed can be produced bypartially eliminating the alloy layer on at least one precoated face ofthe plate by means of a laser beam.

The invention also consists in a welded blank obtained by butt-weldingat least two plates according to any of the above embodiments, thewelded joint being produced on the edge contiguous with the area fromwhich the metal alloy has been removed.

The invention further consists in a part obtained by heat treatment anddeformation of a welded blank according to the above embodiment, theprecoat being converted throughout its thickness by the heat treatmentinto an intermetallic alloy compound providing protection againstcorrosion and decarburization of the steel substrate.

The invention further consists in a plate, blank or part according toany of the above embodiments, the composition of the steel comprising,by weight: 0.10%≦C≦0.5%, 0.5%≦Mn≦3%, 0.1%≦Si≦1%, 0.01%≦Cr≦1%, Ti≦0.2%,Al≦0.1%, S≦0.05%, P≦0.1%, 0.0005%≦B≦0.010%, the remainder consisting ofiron and inevitable impurities resulting from the production process.

The composition of the steel preferably comprises, by weight:0.15%≦C≦0.25%, 0.8%≦Mn≦1.8%, 0.1%≦Si≦0.35%, 0.01%≦Cr≦0.5%, Ti≦0.1%,Al≦0.1%, S≦0.05%, P≦0.1%, 0.002%≦B≦0.005, the remainder consisting ofiron and inevitable impurities produced by the production process.

The invention further consists in a part according to any one of theabove embodiments wherein the microstructure of the steel ismartensitic, bainitic or bainitic-martensitic.

The invention further consists in a method wherein:

-   -   a steel plate is coated to obtain a precoat consisting of an        intermetallic alloy layer topped by a metal alloy layer,    -   on at least one face of the plate the metal alloy layer is        removed in an area at the periphery of the plate.

The width of the area is preferably between 0.2 and 2.2 mm.

The invention further consists in a method of fabricating precoatedsteel plate wherein:

-   -   a steel plate is coated to obtain a precoat consisting of an        intermetallic alloy layer topped by a metal alloy layer, then    -   on at least one face of the plate the metal alloy layer is        removed in an area not totally contiguous with the periphery of        the plate, then    -   the plate is cut in a plane so that the area from which the        metal alloy has been removed is at the periphery of the cut        plate.

The width of the area from which the metal alloy has been removed andwhich is not totally contiguous with the periphery of the plate ispreferably between 0.4 and 30 mm.

The precoating is preferably effected by dip coating with aluminum.

The layer is preferably removed by brushing.

In a preferred embodiment the layer is removed by the impact of a laserbeam on the precoat.

The invention also consists in a method according to any one of theabove embodiments in which the emissivity or reflectivity of the areaover which the metal alloy layer is removed is measured, the measuredvalue is compared with a reference value characteristic of theemissivity or reflectivity of the metal alloy layer, and the removaloperation is stopped when the difference between the measured value andthe reference value is above a critical value.

The invention also consists in a method wherein the layer is removed bymeans of a laser beam, characterized in that the intensity or wavelengthof the radiation emitted at the point of impact of the laser beam ismeasured, the measured value is compared with a reference valuecharacteristic of the emissivity of the metal alloy layer, and theremoval operation is stopped when the difference between the measuredvalue and the reference value is above a critical value.

The invention also consists in a method wherein at least two platesfabricated according to any one of the above embodiments arebutt-welded, the welded joint being produced on the edge contiguous withthe area from which the metal alloy layer has been removed.

The width before welding of the area from which the metal layer has beenremoved at the periphery of the plate is preferably 20 to 40% greaterthan half the width of the weld.

The width of the area from which the metal alloy as been removed andwhich is not totally contiguous with the periphery of the plate ispreferably 20 to 40% greater than the width of a weld.

The invention also consists in a part fabrication method wherein awelded blank fabricated according to the above embodiment is heated toform, by alloying between the steel substrate and the coating, anintermetallic alloy compound, and so as to confer a partially or totallyaustenitic structure on the steel, then

-   -   the blank is hot deformed to obtain a part,    -   the part is cooled at a rate adapted to confer the target        mechanical characteristics.

The rate of cooling is preferably above the critical rate formartensitic quenching.

In a preferred embodiment the welding is effected by a laser beam.

The welding is even more preferably effected by an electrical arc.

The invention also consists in the use of plate, blank or part accordingto any one of the above embodiments for the fabrication of structural orsafety parts for motorized terrestrial automotive vehicles.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

Other features and advantages of the invention will become apparent inthe course of the description given hereinafter by way of example andwith reference to the following appended figures:

FIG. 1 is a diagram showing one embodiment of plate according to theinvention before welding.

FIG. 2 is a diagram of a second embodiment of plate according to theinvention.

FIG. 3 is a diagram of an example of a butt-welded joint of theinvention.

FIG. 4 is a macrograph of a welded joint of the invention afteraustenizing and alloying heat treatment.

FIG. 5 is a macrograph of a reference welded joint showing theappearance of harmful intermetallic areas within the molten metal.

FIG. 6 is a macrograph of plate according to the invention beforewelding, from which plate the metal alloy has been removed locally usinga laser beam.

DETAILED DESCRIPTION OF THE INVENTION

As explained above, total elimination of the metal coating on eitherside of the joint before welding has led to localized corrosionproblems. The inventors have surprisingly shown that eliminating aprecise portion of the coating solves the problems referred to above.

To explain the invention, there are explained first certaincharacteristics of coated strip or plate usually produced by immersionin baths of molten zinc or aluminum or zinc or aluminum alloys.

These continuous, so-called “dip” methods yield the following generalmorphology of the coatings:

-   -   At the surface of the steel substrate of the plate an        intermetallic alloy a few micrometers thick precipitated, formed        by a very fast reaction on immersion in the molten bath. These        intermetallic alloys being relatively fragile, inhibitors are        added to the molten bath in an attempt to limit the growth of        this layer. In the case of zinc or aluminum alloy coatings, the        alloys constituting this layer are often of the Fe_(x)Al_(y)        type, in particular Fe₂Al₅. In the case of zinc alloy coatings,        the presence of this aluminum-rich intermetallic Layer is        explained by the fact that the zinc baths often contain a small        quantity of aluminum that plays an inhibitor role.

This layer of intermetallic alloys can sometimes be of a complex nature,for example divided into two intermetallic sub-layers, the sub-layer incontact with the substrate being richer in iron.

-   -   This layer of intermetallic alloys is topped by a metal alloy        layer the composition of which is very close to that of the        bath. A thicker or thinner metal layer is ent rained by the        plate as it leaves the molten bath, and this thickness can be        controlled by means of jets of air or nitrogen.

The inventors have shown that it is necessary to eliminate this layerlocally to solve the problems referred to above, which is particularlyadvantageous.

Consider more particularly FIG. 1, showing a plate of the invention. Theterm plate is to be understood in a broad sense and denotes inparticular an strip or object obtained by cutting a strip, a coil or asheet. In this particular example the plate has two faces and fouredges. The invention is not limited to this rectangular geometry, ofcourse. FIG. 1 shows:

-   -   A steel substrate 1. This substrate can be of plate that is        hot-rolled or cold-rolled, as a function of the required        thickness, or of any other appropriate form.    -   Superposed on the substrate, and in contact therewith, a        pre-coating 2 is present on the two faces of the part. This        pre-coating itself consists or:        -   a layer of intermetallic alloy 3 in contact with the            substrate 1. As already explained, this layer is formed by            reaction between the substrate and the molten metal of the            bath.

The precoat is advantageously an aluminum alloy or aluminum-based. Thistype of precoat is particularly suitable for subsequent heat treatmentthat forms an intermetallic compound by interdiffusion with thesubstrate 1 and (see below) localized removal of the surface layer. Inparticular, the metal alloy of the precoat can contain 8 to 11% byweight of silicon and 2 to 4% of iron, the remainder consisting ofaluminum and inevitable impurities. Adding silicon enables reduction ofthe thickness of the intermetallic layer 3.

-   -   The periphery 5 of the plate is also shown. According to the        invention, a portion 6 of the periphery does not carry the metal        alloy layer 4 but retains the intermetallic alloy layer 3. This        portion 6 is intended to be placed in contact with another plate        and then to be butt-welded in a plane defined by the edge 11 to        form a blank.    -   In a first embodiment, the layer 4 is advantageously removed by        means of a brushing operation effected at the periphery 5: the        material removed by the brush is essentially the surface layer,        which has the lowest hardness, i.e. the metal alloy layer 4. The        harder layer 3 will remain in place as the brush passes over it.        Using an aluminum or aluminum-based precoat is particularly        advantageous as the difference in hardness between the        intermetallic alloy layer 3 and the metal layer 4 is very large.

The person skilled in the art will know how to adapt the variousparameters specific to the brushing operation, such as the choice of thekind of brush, the speed of rotation and of relative movement antranslation, the pressure perpendicular to the surface, to carry out theremoval as completely and quickly as possible, adapting them to theparticular nature of the precoat. For example, a wire brush mounted on arotary shaft driven in translation parallel to the edge of the part 6could be used.

-   -   In a second embodiment, the layer 4 is removed by a laser beam        directed toward the periphery of the plate: interaction between        this high energy density beam and the precoat causes        vaporization and expulsion of the surface of the precoat. Given        the different thermal and physical properties of the metal alloy        layer 4 and the intermetallic layer 3, the inventors have shown        that a success ion of short laser pulses with appropriate        parameters leads to selective ablation of the metal layer 4,        leaving the layer 3 in place. The interaction of a pulsed laser        beam directed toward the periphery of a coated plate and moved        in translation relative to that plate therefore removes the        peripheral metal layer 4. The person skilled in the art will        know how to adapt the various parameters, such as the choice of        laser beam, the incident energy, the pulse duration, the speed        of relative movement in translation between the beam and the        plate, and the focusing of the beam onto the surface to carry        out the ablation as quickly and completely as possible, adapting        them to the particular nature of the precoat. For example, a        Q-switch laser could be used, having a nominal power of a few        hundred watts and delivering pulses with a duration of the order        of 50 nanoseconds. The width of the removal area 6 can naturally        be varied by means of successive contiguous ablations.

The width of the area 6 from which the metal layer has been removed mustbe adjusted to enable:

-   -   welding with no introduction of any element of the precoat into        the molten area,    -   sufficient corrosion resistance of the welded assembly after        subsequent alloying and austenizing heat treatment.

The inventors have shown that the above conditions are satisfied if thewidth of the area 6 is 20% to 40% greater than half the width of themolten area created when butt-welding blanks.

The minimum value of 20% ensures that the precoat is not introduced intothe molten metal during welding, and the value of 40% ensuressatisfactory corrosion resistance.

Given the welding conditions for plate from 1 to 3 mm thick, the widthof the area 6 is between 0.2 and 2.2 mm.

This situation is represented in FIG. 3, which shows diagrammatically insection after welding plate comprising a precoat 2 formed of anintermetallic alloy layer 3 and a metal layer 4. The molten area 10 hasits axial plane 9 in the welding direction. The dashed lines show theinitial extent of an area 6 melted by the welding operation.

FIG. 3 illustrates the situation in which the weld is globallysymmetrically on the two opposite faces of the plate. Under theseconditions, the width of the area 6 is exactly the same on both faces.However, as a function of the welding process used and the parameters ofthat process, the weld can have an asymmetrical appearance. According tothe invention, the width of the area 6 can then be coordinated to thisasymmetry so that this width is slightly greater than half the width ofthe molten area 10 on each of the respective two faces. Under theseconditions, the width of the area 6 differs from that of the area 6shown in FIG. 3.

If welding conditions evolve during an assembly operation, for exampleto take account of local modification of geometry or thickness, thewidth of the area 6 can also be coordinated with the correspondingvariation of the width of the molten area along the welded periphery ofthe plate. The width of the area 6 naturally increases if localconditions lead to the formation of a wider weld.

In the case of welding two coated plates of different thickness, thewidth of the area 6 can also be different on the welded peripheralportion of each of the two plates.

In a variant of the invention shown in FIG. 2, the layer 4 is removedover an area 7 of a coated plate that is not totally contiguous with theperiphery 5 of the plate. The plate is then cut in an axial plane 8perpendicular thereto, for example by a slitting process. A plate asshown in FIG. 1 is then obtained. The width removed is 20% to 40%greater than the width of the molten area that would be produced by awelding operation in the axial plane 8.

In one variant of the invention, the width removed is between 0.4 and 30mm. The minimum value corresponds to a width such that cutting in theaxial plane 8 produces two plates having a very narrow removal area 0.2mm wide on each of the two plates. The maximum value of mm correspondsto a removal width well suited to industrial tools for performing suchremoval. A subsequent cutting operation can be effected, not on theaxial plane 8 situated in the middle of the removal area, but at alocation adapted to produce a plate whose removal width is slightlygreater than half the width of the molten area produced by a weldingoperation, defined by the conditions of the invention.

As explained above, the removed widths ensure that the metal coating isnot introduced into the molten metal during subsequent welding of theplate and also that the welded blank is corrosion resistant after heattreatment.

Removal of the metal layer 4 can be monitored by means of micrographicexamination. However, it has also been shown that the efficiency of theremoval operation can be checked very quickly by optical inspection:there is a difference in appearance between the metal layer 4 and theunderlying intermetallic layer 3, which is darker. The removal operationmust therefore continue and be stopped when there is seen in the area 6a significant change of tone relative to the surface coating. It istherefore possible to monitor removal by spectrometer reflectivity oremissivity measurement: the area 6 is illuminated by a light source, oneor more optical sensors being directed towards this area. The measuredvalue corresponds to the reflected energy. That value is compared with areference value corresponding to the emissivity or reflectivity of themetal layer 4 or with a value measured by another sensor directed towardthe metal layer. It is also possible to measure the variation of thereflected energy as a function of time. If the layer 6 is flush with thesurface, the energy collected is lower than that corresponding to themetal alloy layer 4. The precise moment at which the removal operationreaches the layer 3 can therefore be determined by previous calibration.

In the case of coating removal by laser ablation, it is also possible toanalyze the intensity or the wavelength of the radiation emitted at thepoint of impact of the laser beam on the precoated plate. The intensityand the wavelength are modified when the layer 4 has been eliminated andthe laser beam impacts on the layer 3. The thickness of the layerremoved can therefore be monitored in the following manner: theintensity or the wavelength of the radiation emitted at the point ofimpact of the laser beam is measured, that measured value is comparedwith a reference value characteristic of the emissivity of the metalalloy layer 4, and the removal operation is stopped when the differencebetween the measured value and the reference value is above aredetermined critical value.

Depending on specific constraints, this step of removing the metal alloylayer can be carried out at various stages of the production process,and in particular:

-   -   either after unwinding coils fabricated on continuous rolling        mill trains, before cutting to form a smaller format plate,    -   or before welding the cut plate.

In the method of the invention, a hot- or cold-rolled steel plate withthe following composition by weight is the starting material: carboncontent between 0.10 and 0.5%, and preferably between 0.15 and 0.25% byweight. This element impacts greatly on the quenchability and on themechanical strength obtained after cooling that follows the alloying andaustenizing of the welded blanks. Below a content of 0.10% by weight,the quenchability is too low and the strength properties areinsufficient. In contrast, beyond a content of 0.5% by weight, the riskof defects appearing during quenching is increased, especially for thethickest parts. A carbon content between 0.15 and 0.25% produces atensile strength between about 1250 and 1650 MPa.

-   -   Apart from its role as a deoxidant, manganese also has a        significant effect on quenchability, in particular if its        concentration by weight is at least 0.5% and preferably 0.3%.        However, too great a quantity (3% by weight, or preferably 1.8%)        leads to risks of excessive segregation.    -   The silicon content of the steel must be between 0.1 and 1% by        weight, and preferably between 0.1 and 0.35%. Apart from its        role of deoxidizing the liquid steel, this element contributes        to hardening. Its content must nevertheless be limited to avoid        excessive formation of oxides and to encourage coatability.    -   Beyond a content above 0.01% chromium increase quenchability and        contributes to obtaining high strength after the hot forming        operation, in the various portions of the part after cooling        following the austenizing and alloying heat treatment. Above a        content equal to 1% (preferably 0.5%), the contribution of        chromium to obtaining homogeneous mechanical properties reaches        saturation.    -   Aluminium favors deoxidation and precipitation of nitrogen. In        amounts above 0.1% by weight, coarse aluminates form during        production, which is an incentive to limit the content to this        value.    -   Excessive quantities of sulfur and phosphorus lead to increased        weakness. For this reason it is preferable to limit their        respective contents to 0.05 and 0.1% by weight.    -   Boron, the content of which must be between 0.0005 and 0.010% by        weight, and preferably between 0.002 and 0.005% by weight, has a        large impact on quenchability. Below a content of 0.005%,        insufficient effect is achieved vis à vis quenchability. The        full effect is obtained for a content of 0.002%. The maximum        boron content must be less than 0.010%, and preferably 0.005%,        in order not to degrade toughness.    -   Titanium has a high affinity for nitrogen and therefore        contributes to protecting the boron so that this element is        found in free form to have its full effect on quenchability.        Above 0.2%, and more particularly 0.1%, there is however a risk        of forming coarse titanium nitrides in the liquid steel, which        have a harmful effect on toughness.

After preparation of the plate according to any of the methods describedabove, they are assembled by welding to obtain a welded blank. More thantwo plates can naturally be assembled to fabricate complex finishedparts. The plates can be of different thickness or composition toprovide the required properties locally.

Welding is effected after placing the plates edge-to-edge, the areaswith no metal alloy layer being in contact with each other. Welding istherefore effected along the edge contiguous with the areas 6 where themetal alloy layer has been removed.

In the context of the invention, any continuous welding means can beused appropriate to the thicknesses and to the productivity and qualityconditions required for the welded joints, and in particular:

-   -   laser beam welding,    -   electric arc welding, and in particular the GTAW (Gas Tungsten        Arc Welding), plasma, MIG (Metal Inert Gas) or MAG (Metal Active        Gas) processes.

Under the conditions of the invention, the welding operation does notlead to remelting of a portion of the metal coating 4, elements whereofwould thereafter be found in the molten area. Only a minimal quantity ofthe intermetallic alloy layer 3 is remelted by this operation into themolten area. As the following example shows, this very limited quantityhas no influence on the metallurgical quality or the mechanicalproperties of the welded joint after alloying and austenizing heattreatment.

The welded blank is then heated to bring about conjointly:

-   -   A surface alloying treatment in which elements of the steel,        substrate, in particular iron, manganese and silicon, diffuse        into the precoat. This forms a surface intermetallic alloy        compound the melting point of which is significantly higher than        that of the metal alloy layer 4. The presence of this compound        during heat treatment prevents oxidation and decarburization of        the underlying steel,    -   Austenizing of the base steel, either partial or total. The        heating is advantageously effected in a furnace so that the part        reaches a temperature between Ac1 and Ac3+100° C. Ac1 and Ac3        are respectively the start and end temperatures of the        austenitic trans format ion that occurs on heating. According to        the invention, this temperature is maintained for a time greater        than or equal to 20 s so as to render uniform the temperature        and microstructure at the various points of the part.

Under the conditions of the invention, during this heating phase, nobrittle intermetallic areas are formed within the molten metal, whichwould be harmful to the mechanical properties of the part.

This is followed by hot deformation of the blank to its final shape as apart, this step being favored by the reduction of the creep limit andthe increase of the ductility of the steel with temperature. Startingfrom a structure that is partly or totally austenitic at hightemperature, the part is then cooled under appropriate conditions toconfer the target mechanical characteristics: in particular, the partcan be held in a tooling during cooling, and the tooling can itself becooled to encourage the evacuation of heat. To obtain good mechanicalproperties, it is preferable to produce martensitic, bainitic orbainitic-martensitic microstructures.

In the area 6 on either side of the welded joint, the intermetalliclayer 3, which is between 3 and 10 micrometers thick before heattreatment, is alloyed with the steel substrate and produces goodcorrosion resistance.

EXAMPLE

The following embodiments show by way of example other advantagesconferred by the invention. They concern a cold-rolled steel strip 1.5mm thick, with the following composition by weight:

TABLE 1 Composition of the steel (% by weight) C Mn Si S P Al Cr Ti B0.224 1.160 0.226 0.005 0.013 0.044 0.189 0.041 0.0031

The steel strip was precoated by dipping it in a molten bath of analuminum alloy containing 9.3% of silicon and 2.8% of iron, theremainder consisting of aluminum and inevitable impurities. The stripwas then cut into plates with a format of 300×500 mm². These have oreach face a precoat comprising a layer of intermetallic alloy comprisingmostly Fe₂Al₃, Fe₂Al₅ and Fe_(x)Al_(y)Si_(z). This 5 micrometers thicklayer in contact with the steel substrate has a 20 micrometers thicklayer of Al—Si metal alloy on top of it.

Before laser beam welding, four different preparation methods were used:

-   -   Method I (according to the invention): the Al—Si metal alloy        layer was removed by longitudinal brushing over a width of 1.1        mm from the edge of the plate, on the 500 mm long side. Brushing        was effected in exactly the same way on both faces using an 80        mm diameter “Spiraband” wire brush mounted on an angled rotary        system, guided in movement in translation on a counterweight        bench. The brushing force is approximately 35 N at the point of        brush/blank contact, and the speed of movement of the brush 10        m/min. This brushing eliminates the metal alloy layer, leaving        only the 5 micrometer intermetallic alloy layer in the brushed        area.    -   Method II (according to the invention): the Al—Si metal alloy        layer was removed by laser ablation over a width of 0.9 mm from        the edge of the plate. The laser ablation was carried out in        exactly the same way on both faces using a Q-switch laser with a        nominal energy of 450 W delivering 70 ns pulses. The pulse        energy is 42 mJ. The constant speed of movement in translation        of the laser beam relative to the plate is 20 m/min. FIG. 6        shows that this laser ablation eliminates the metal alloy layer        4 leaving only the 5 micrometer intermetallic alloy layer 3 in        the treated area.    -   Method R1 (not according to the invention) all of the precoat        comprising the metal alloy layer and the intermetallic alloy,        was mechanically removed over a width of 1.1 mm, and therefore        identical to that of method 1, by means of a carbide plate type        tool for fast machining, in longitudinal translation. As a        result, subsequent welding is carried out in an area with all of        the precoat removed on either side of the joint.    -   Method R2 (not according to the invention): laser welding was        effected on precoated plate with no particular preparation of        the periphery.

The above plates were laser beam welded under the following conditions:nominal power: 6 kW, welding speed: 4 m/minute. Given the width of theweld, in method I, there is found the presence of an area with no metalalloy over a width of approximately 0.3 mm following production of thewelded joints.

The welded blanks were subjected to alloying and austenizing heattreatment including heating to a temperature of 920° C., which wasmaintained for 7 minutes. These conditions lead to complete austenitictransformation of the steel of the substrate, During this heating andconstant temperature phase, it is found that the aluminum-silicon-basedprecoat forms an intermetallic compound throughout its thickness byalloying with the base steel. This alloy coating has a high meltingpoint and a high hardness, features high corrosion resistance, andprevents oxidation and decarburization of the underlying base steelduring and after the heating phase.

After the phase of heating to 920° C., the parts were hot-deformed andcooled.

Subsequent cooling between jigs yielded a martensitic structure. Thetensile R_(m) of the steel substrate obtained after such treatment isabove 1450 Mia.

The following techniques were then used to characterize the weldedjoints in the parts obtained in this way:

-   -   Micrographic sections show the presence of any intermetallic        areas within the welded joints.    -   Mechanical tension tests across welded joints in samples 12.5×50        mm² determines the tensile strength R_(m) and the total        elongation.    -   Accelerated corrosion tests were carried out according to the        DIN 50021, 50017, and 50014 standards. These tests include,        following salt mist spraying, cycles alternating dry phases at        23′ and wet phases at 40° C.

Table 2 sets out the results of these characterizations:

TABLE 2 Welded joint characteristics after heat treatment Fragileintermetallic areas within Corrosion Method welded joints Rm (MPa) A(%)resistance I None >1450 ≧4 ◯ (according to the invention) II None >1450≧4 ◯ (according to the invention) R1 (not None >1450 ≧4 ● according tothe invention) R2 (not Present 1230 ≦1 ◯ according to the invention) ◯:Satisfactory ●: not satisfactory

Under the quenching conditions required after heat treatment, themicrostructure of the base metal and the molten area during welding istotally martensitic with the above four methods.

In the case of method I of the invention, the melted area contains nointermetallic area, as FIG. 4 shows.

On the other hand, in the method R2, note the presence of intermetallicareas (see FIG. 5) in particular towards the periphery the of the meltedarea where the elements of the precoat were concentrated by spontaneousconvection currents in the liquid bath caused by a Marangoni effect.These large intermetallic areas, which can be oriented substantiallyperpendicularly to the mechanical load, act as stress concentration andonset of rupture effects, Elongation in the crosswise direction is inparticular reduced by the presence of these intermetallic areas in theabsence of these areas, the elongation is above 4%. It drops to below 1%when they are present.

No significant difference in mechanical characteristics (strength andelongation) is noted between the method I of the invention and themethod R1. This indicates that the thin layer of intermetallic alloyleft in place by brushing and remelted by welding does not lead to theformation of brittle areas within the molten metal, as FIG. 4 shows.

In the case of the method R1, corrosion resistance is reduced: the steelis totally bared on either side of the welded joint by the total removalof the precoat. Lacking corrosion protection, red rust is then seen toappear in the heat-affected areas on either side of the weld.

Thus the method of the invention simultaneously achieves good ductilityof the welded joint after treatment and good corrosion resistance.

Depending on the composition of the steel in particular its carboncontent and its manganese, chromium and boron content, the maximumstrength of the parts can be adapted to the target use. Such parts willbe used with profit for the fabrication of safety parts, and inparticular anti-intrusion or underbody parts, reinforcing bars,B-pillars, for the construction of automotive vehicles.

The invention claimed is:
 1. A method of fabricating a precoated steelplate, the method comprising: (A) coating a steel plate by dipping thesteel plate in a molten bath to obtain a precoat upon the steel plate,wherein the precoat consists of (i) an intermetallic alloy layer and(ii) a metal alloy layer, wherein the intermetallic alloy layer (i) istopped by the metal alloy layer (ii); and (B) on at least one face ofthe plate, removing the metal alloy layer (ii) in an area at a peripheryof the plate having a width from 0.2 to 2.2 mm, while leaving at leastpart of the intermetallic alloy layer (i) in the area, wherein the metalalloy layer (ii) is removed while leaving all of the intermetallic alloylayer (i) in the area and the metal alloy layer remains in an areaoutside the periphery of the plate.
 2. A method of fabricating aprecoated steel plate, the method comprising: (A) coating a steel plateby dipping the steel plate in a molten bath to obtain a precoat upon thesteel plate, wherein the precoat consists of (i) an intermetallic alloylayer and (ii) a metal alloy layer, wherein the intermetallic alloylayer (i) is topped by the metal alloy layer (ii); and (B) on at leastone face of the plate, removing the metal alloy layer (ii) in an area ata periphery of the plate having a width from 0.2 to 2.2 mm, whileleaving at least part of the intermetallic alloy layer (i) in the areaand leaving the metal alloy layer in an area outside the periphery ofthe plate, wherein the metal alloy layer (ii) has a lower hardness thanthe intermetallic alloy layer (i), the metal alloy layer (ii) is removedby means of a brushing operation and the removal is a function of thehardness.